Some circuits such as analog receivers are able to process input signals only within certain bounds of voltage, known as the input voltage range of the receiver. If an input signal is outside this range, then signal distortion and other undesirable effects occur. For example, power supply noise may be reflected in a received signal and may cause the received signal to be outside the input voltage range of the receiver's amplifiers. In this situation, it is necessary to somehow limit the received signal to upper and lower boundaries.
There are several known techniques to limit the amplitude of an input signal to the input voltage range of the amplifier. One technique compares the voltage on the input signal line with two reference voltages, corresponding to upper and lower limits of the input voltage range. If the input signal voltage exceeds the upper limit, the output of a first comparator causes a first reference voltage to be switched in, effectively clipping the input signal at the upper limit. Similarly, when the input signal voltage is less than the lower limit, a second comparator clips the input signal to a second reference voltage, effectively clipping the input signal at the lower limit. The disadvantage of this approach is that the two comparators require a significant amount of integrated circuit area.
Another voltage limiter, such as that disclosed by G. Brehmer and H. Jackson in U.S. Pat. No. 4,849,708, provides clamping transistors in feedback paths of a differential amplifier. However, this technique requires an additional differential amplifier, which consumes significant integrated circuit area and thus may not be desirable for cost-sensitive applications. Also, inserting an additional amplifier in the input signal path reduces the effective bandwidth in the receiver. In addition, such a circuit may be sensitive to power supply noise.